1. Field
The present invention relates generally to altimeters and methods for providing altitude information.
2. Background
Unmanned aerial vehicles (UAVs) play an increasingly valuable role in a wide range of commercial, governmental, military, and science applications. In comparison to manned aircraft, UAVs offer many advantages, including reduced initial and operating costs, versatile basing and storage options, reduced visible and radar signatures, ease of transport, and increased suitability for operation in hazardous environments. However, despite these advantages, various limitations have decreased the acceptance and adoption of UAVs.
One such limitation is the need for skilled operators to launch and, more importantly, land the aircraft. Pilots, or “operators”, of UAVs are often trained using flight simulation programs and actual flying or handling time. Novice operators are more likely to crash or make other mistakes during actual launching and landing training. Unlike a manned aircraft in which the pilot uses visual markers, a UAV operator must rely on information sensed by the UAV, relayed to the control and navigation processing system, and displayed to the operator. Operators often have difficulty anticipating or avoiding low altitude obstacles, and must rely on the UAV's control and navigation processing system to accurately identify the obstacles and display the information in a form that is useful to the operators. Alternatively, UAVs may be landed by distant operators using visual cues to infer altitude, attitude, and other relevant parameters.
Consumer global positioning systems (GPSs) have an average uncertainty of three meters, and, as such, a UAV navigation and control system is needed for altitudes below approximately six meters. Thus, the operating range of such a system is from ground level (approximately zero meters) to six meters above the local, flight path terrain. Known navigation and altimeter technologies include radar systems, lidars, acoustic sensors, and infrared range sensors, each of which offers unique advantages and disadvantages. Radar systems offer all-weather capability and immunity to visual obstacles such as smoke, haze, and fog, but can be susceptible to jamming, can be confused by low-reflectivity surfaces (e.g., dry sand or snow), and can be relatively heavy. An example of a small radar system is Roke Manor Research Ltd.'s Miniature Radar Altimeter Type 2, which operates at a frequency of 77-GHz frequency with an update rate of 10 Hz, and has range accuracy of 2 cm for ranges from 20 cm to 100 m. This system, with its integrated antennas, measures 5.5 in.×3 in.×1.8 in., weighs 14.1 oz., and consumes about 3 W.
Lidars offer fine range accuracy, but, unless they are gimbaled or utilize beam scanning, their measurements of height may be corrupted by unknown aircraft attitudes. An example of a non-scanning lidar system is Laser Technology Inc.'s Universal Laser Sensor (ULS), which operates at a wavelength of 905 nm with a range accuracy of 2 cm for ranges from 46 cm to 500 m. This system measures 5.3 in.×4.7 in.×2.5 in., weighs 28.2 oz., and consumes about 2 W.
Acoustic sensors are compact and accurate, but their performance may be degraded by noise due to wind, turbulence, vibration, or engines (on the host vehicle or a nearby vehicle). Additionally, acoustic sensors may interfere with other aircraft systems through electromagnetic or radio-frequency coupling. An example of an acoustic sensor is Devantech's SRF08 High Performance Ultrasonic Range Finder, which operates at a frequency of 40 kHz and provides a range accuracy of 3 cm over a 3 cm to 6 m. This low-cost component measures 1.7 in.×0.8 in.×0.7 in., weighs 0.4 oz., and consumes about 0.1 W. However, the power, processing, or external interfacing systems are not included in the size, weight, or power specifications.
Infrared range sensors are limited by the working distances of the sensors. HeliCommand's Profi-series senses platform motion to enable stabilization, its four optical imaging systems analyze scene features in images collected using ambient lighting, it uses a 3-axis accelerometer and gyroscopes, and a barometric altimeter provides altitude. This system operates at heights up to 30 m. This system, which was designed for radio-controlled helicopters, measures 2.9 in. height×2.9 in. diameter, weighs 8.1 oz., and consumes approximately 1 W. An optional infrared range sensor provides altitude data up to 1.5 m above local terrain with a range accuracy of 10 cm.
Some prior art systems measure round-trip time-of-flight to determine target range. Precise range measurements under this approach require combinations of modulated microwave, millimeter-wave, or optical sources in conjunction with high-speed electronics or radio-frequency (RF) signal processing, which drives system complexity and cost. Furthermore, the microwave, millimeter-wave, or radio signals may interfere with other sensing or communication equipment.
To assist with spacecraft docking maneuvers, NASA has developed an Advanced Video Guidance Sensor (AVGS) sensor that includes a laser illuminator, retroreflectors, and a camera array coupled to signal processors to determine target range and bearing. The AVGS provides a range accuracy of about 1% of the measured range for ranges from less than 1 m to beyond 100 m. However, the AVGS weighs 20 pounds and requires retroreflectors, both of which make it impractical for use on small UAVs. Other NASA laser and bearing finders involve time-of-flight measurement to determine target range, and are also not appropriate for use on small UAVs due to their relatively high weight and cost
Thus, there is a need for an improved system and method to better enable persons, especially those with minimal training, to avoid low altitude obstacles during the launch, flight, and landing of UAVs.